It helps having knowledge of what every champion does. So if you're the jungler and your team locked in Riven top, Zed mid, Draven ADC and Janna support what do you see? I see a very heavy AD centric team who likes killing. The team could do wth some AP damage output who can grant the team some much needed CC. It wouldn't hurt to be tanky either. If I saw this I would consider Gragas, Fiddlesticks, Diana and Nautilus as some of the better picks. So I saw what my team lacked (AP damage) and saw what strength my team was a heavy kill focused team so I wanted something with good poke and crowd control with initiation capabilities. This gives Draven the kills he wants, Zed the opportunity to make pick offs for the enemy back liners and takes some of the pressure off Janna in having to initiate via tornado allowing her to do what she does best: peel.
Now if you want to counter the enemy team you should get familiar with the three play styles of this game. They are poke, kill and sustain. They work much like a rock, paper and scissors algorithm. Poke beats kill, kill beats sustain and sustain beats poke. Every champ has one of these styles they excel at, one they are mediocre with and one they have little or no ability to pull off effectively. Someone like Zed is a kill/poke style champion. His hardest counters are poke/sustain laners. Someone like Vladimir fits the profile nicely. Vladimir can poke Zed back when Zed pokes him and can mitigate the damage through the healing aspects of his transfusion. It also helps that Vlad is manaless too. Now if you want to counter Vladimir who is poke/sustain you would want to choose someone who is sustain/kill based. The hardest counter to Vladimir is Swain because Swain has his own sustain and a healing debuff spell. Swain also has more consistent damage output through his ultimate. Now lets see you try. Swain is a sustain/kill laner. You want someone who is a kill/poke style laner. Besides Zed who do you pick as a counter to Swain?